Development of arable weed seedbanks during the 6 years after the change from conventional to organic farming H ALBRECHT Vegetation Ecology, Department of Ecology, TU Muenchen-Weihenstephan, Freising, Germany Received 10 March 2004 Revised version accepted 18 May 2005 Summary All of the arable land of a farm in southern Germany was investigated during an 8-year period, starting 2 years before and ending 6 years after the change to organic farming. The first 3 years after the conversion, the total seed number in the soil increased from 4050 to 17 320 m )2 . From the fourth to the sixth year, it dropped back to 10 220 m )2 . The number of seeds increased particularly at sites with a low crop cover and a high density of weed plants at the soil surface. The increase predominantly occurring on the more fertile soils may have been caused by the rotation commencing with less competitive crops. Crops which increased the seedbank by 30–40% were winter cereals, sunflowers and lupins. Potatoes and sown fallow caused no significant change and grass–clover mixtures even reduced the number of seeds by 39%. Among 44 species occurring frequently enough for statistical analysis, 31 increased and only 3 decreased. The change of manage- ment particularly increased summer annual, perennial and dicotyledonous weeds. This can be attributed to both operations which are characteristic of organic farming (e.g. replacing herbicide applications by mechanical weed control) and to general modifications of the management practice which may also occur in non-organic farming systems (increasing the percentage of broad leaved and spring sown crops in crop rotation). The present study confirms investigations into the aboveground vegetation that indicate that arable or- ganic farming favours plant species diversity and pro- vides evidence that the conversion need not encourage the dominance of a few noxious weeds. Keywords: seed number, weed density, weed cover, crop cover, crop rotation, soil, biodiversity, dominance. ALBRECHT H (2005) Development of arable weed seedbanks during the 6 years after the change from conventional to organic farming. Weed Research 45, 339–350. Introduction Both to reduce environmental problems which arise from continuous intensive exploitation and to provide food without pesticide residues, governments of the European Community countries currently foster organic farming as an option for a sustainable land use (European Communities, 2002). Thus, the percentage of farmland under organic management in the EC member countries increased from 0.5%, when the corresponding Regulation EC 2078/92 was released in 1992, to a level of 3.3% by the end of 2001 (Commission of the European Communities, 2002). A common observation in studies from England (Moreby et al., 1994), Denmark (Hald, 1999), Sweden (Rydberg & Milberg, 2000), Finland (Salonen et al., 2001), Austria (Plakholm, 1989) and Germany (Frieben & Ko¨pke, 1996; van Elsen, 2000) was that species diversity increased when arable farming was changed from conventional to organic management. In addition, positive effects were recorded for the rare species. Conversion to organic farming, however, does not only Correspondence: H Albrecht, Vegetation Ecology, Department of Ecology, TU Muenchen-Weihenstephan, 85350 Freising, Germany. Tel.: +49- 8161/713717; Fax: +49-8161/714143; E-mail: albrecht@wzw.tum.de Ó 2005 European Weed Research Society Weed Research 2005 45, 339–350